Model for the contribution of Snf7p and other ESCRT-III components to BMV spherule formation.

<p>(A) BMV 1a multimerization initiates perinuclear ER membrane invagination away from the cytoplasm, followed by recruitment of ESCRT proteins away from MVBs to the site of BMV RNA replication. (B) The concentric spirals formed by ESCRT-III components serve to constrict the wide membrane rim induced by 1a to bring the membrane lipids into close proximity to form the neck of the vesicle. (C) Unlike its natural role in MVBs where ESCRT-III continues to constrict the vesicle neck to efficiently cleave the bud to form intralumenal vesicles, either 1a, the reticulons (Rtn), which have been previously implicated in stabilizing spherule necks, or an unidentified protein prevent ESCRT-III from achieving membrane fusion, leaving an intact neck structure. (D) The ATPase Vps4p uses the energy of ATP hydrolysis to catalyze the disassembly of the ESCRT machinery and the recycling of its subunits from the membrane, leaving a spherule neck stably attached to the ER membrane.